Test systems are equipped by their manufacturer with a certain number of output channels that enable a certain number of chips to be tested in parallel. For example, as illustrated in the enclosed FIG. 1, a test signal from a transmit driver (DRV) is distributed in parallel to four functionally identical pins X of four chips under test via a test channel having a line impedance of 50 Ohms.
In order to save costs, one always aims to increase parallelism, to which end one DRV is no longer connected to one pin of a chip but is connected in parallel to functionally identical pins of a plurality of chips. This concept is also known as the “shared driver” concept. For example, a memory tester from Advantest, type T5571P, contains 480 transmit drivers and 288 IOs for 16 DUTs (Devices Under Test), where 30 DRVs and 18 IOs are used for each DUT.
In the test arrangement illustrated in FIG. 1, based on the shared driver concept, the inputs of the chips under test do not draw more current in total than the DRV can supply. The line impedance or output impedance of the DRV of 50 Ohms must also be taken into account, because a potential divider is created by the output impedance and the parallel connection of the input resistances Rin1, Rin2, . . . , Rin4 of the chips as illustrated in FIG. 2.
If, as illustrated in FIG. 4, a short-circuit occurs in one of the pins of a chip, indicated by the input impedance of 0 Ohms, the test signal becomes 0 (0 V) and is therefore no longer available for the other chips either. This means that the one faulty chip turns the other potentially good chips into failed chips during testing, reducing the yield. In order to avoid this problem caused by the short-circuit of a chip input or pin, in one design, series resistors are inserted in the separate secondary signal lines (see FIG. 3). This only works for high impedance inputs, however, because this is also a potential divider. FIG. 5 illustrates that when using such series resistors as in FIG. 3, in the worst-case situation when one pin is short-circuited, a three-element potential divider is formed from the 50 Ohm output impedance of the DRV, the 1 kOhm series resistor and the input impedance of the chip. For every short-circuited chip input or pin, there is an incremental reduction in the voltage to the other chips.
Hence it is one object of the invention to make possible using few means, by extending the shared driver concept, a generic test arrangement without loss of yield from potentially short-circuited chips.